Magnetic resonance logging instruments are often used in oil field applications to determine the various properties of an earth formation. The magnetic resonance logging instrument is lowered into a bore-hole and non-intrusively measures the various properties in the different strata of the earth formation. The measurements can determine various petrophysical properties of the fluids in the earth formation, as well as providing information about the porosity, permeability, and wetability in the earth formation. Most importantly, magnetic resonance measurements can be used to separately identify and map the oil, water, and other constituents within the earth formation.
Magnetic resonance logging instruments generally operate by first locally placing the earth formation in a static background magnetic field, in which the unpolarized magnetic moments of the hydrogen atoms in the earth formation precess at a characteristic frequency. A polarizing pulse of radio-frequency power is then applied to the earth formation at this same frequency, causing the magnetic moments of the hydrogen atoms to align in a common direction. The polarizing pulse is then turned off, and the time in which this signal decays is the relaxation time "T1." An antenna is used to detect the radio-frequency signal from the precessing polarization of the hydrogen atoms in the earth formation, and the rate in which this signal decays is the free induction decay "T2." T1 and T2 are a sensitive measure of the physical and chemical environment that the hydrogen atoms are located. For example, T1 and T2 can be used to determine whether the hydrogen atoms are part of a water molecule or in a hydrocarbon, and whether the hydrogen atoms are tightly bound in a micro porous region or part of a larger fluid volume. The pulse-and-listen process described above can be repeated to produce a spin echo of the first pulse, which provides enhanced analyzing power.
Conventional magnetic resonance logging instruments utilize a single permanent magnet within the logging instrument to produce the magnetic field. The permanent magnet only provides a magnetic field strong enough to measure the properties of the earth formation approximately twenty centimeters into the earth formation. At twenty centimeters the properties of the earth formation have been compromised by the drilling operation and drilling fluids utilized to produce the well. Accordingly, conventional magnetic resonance logging instruments are not capable of peering into the earth formation past the zone affected by drilling operations. In addition, conventional magnetic resonance logging instruments must proceed at very slow speeds in order to take measurements. The slow speed drastically increases the amount of time to fully log a well.
In addition, conventional magnetic resonance logging instruments cannot readily distinguish oil from water. In order to distinguish between oil and water, the instrument requires a region of relatively uniform magnetic field within the earth formation. Conventional magnetic resonance logging instruments do not provide such a uniform magnetic field.
Furthermore, conventional magnetic resonance logging instruments cannot readily distinguish between hydrogen atoms from sodium atoms. The resonate frequency of sodium atoms is a multiple of the resonate frequency of hydrogen atoms and causes an aliasing signal that is indistinguishable from the hydrogen signal.